The mechanism of tumorigenesis by a bacterium, Agrobacterium tumefaciens, will be studied. Extracellular infections by this bacterium cause the formation of crown gall tumors. During the course of the infection, bacterial tumor-inducing (Ti) plasmid DNA is transferred from the bacterium to the host cell. These Ti plasmid DNA sequences are maintained and transcribed in the tumor cell. The interaction of the bacterium with tissue culture cells will be studied. Virulent strains of A. tumefaciens attach to the surface of host cells in vivo and in tissue culture. The host cell receptor which binds the bacterium will be characterized. Avirulent bacterial mutants which do not attach to tissue culture cells have been obtained and are being used to characterize the bacterial binding site. Avirulent bacterial mutants blocked at steps in tumor formation subsequent to attachment will be studied in order to elucidate the mechanism of transfer of plasmid DNA from the bacterium to the host cell.